Planetary Systems
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Planetary Systems
The year 1995 has seen a very important discovery. For the first time, a planet was detected that does not belong to our solar system. Since that time, the methods to find extrasolar planets have been refined and up to the year 2008, more than 250 extrasolar planets were listed. Since planets only reflect light from their central star, they are extremely difficult to detect: • The radiation from the central star is orders of magnitudes higher than the radiation of the orbiting planet. • As seen from Earth, the planet is generally quite close to the central star. • These observational difficulties cause strong bias since it is more likely that massive planets will be detected than small faint ones. The discovery of the first extrasolar planet can be compared with the Copernican Revolution that not the Earth but the Sun is the center of our planetary system and the discovery of Hubble, that the universe is expanding. Since 1995 it is evident that planetary systems are common in the universe and this certainly has a big impact on the question of whether there are other habitable planets or not. We start this chapter with a brief overview about our planetary system, the solar system.
4.1 The Solar System: Overview and Formation 4.1.1 Planets — How Are They Defined? What is the difference between a star and a planet? A first answer could be that planets do not shine by themselves, they reflect light from their host star. A more precise definition includes the mass of the objects. • Bodies with masses > 0.075 M reach core temperatures ∼ 107 K where hydrogen fusion sets in. Note that a mass > 0.075 M equals to about 75 times the mass of Jupiter (also written as 75 MJ ). Mass of Jupiter: 1 MJ = 318 ME = 2 × 1027 kg, ME is the mass of the Earth. Stars at the lower mass edge are expected to have a surface temperature of only 2,000 K. A. Hanslmeier, Habitability and Cosmic Catastrophes, Advances in Astrobiology and Biogeophysics, DOI 10.1007/978-3-540-76945-3 4, c Springer-Verlag Berlin Heidelberg 2009
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Planetary Systems
• Brown Dwarfs: their mass range is between 13 MJ and 75 MJ . The distinction between the brown dwarfs and the even less massive planets is that the core temperatures of brown dwarfs can reach 106 K and higher. Thus, short intervals of nuclear burning of deuterium and lithium are likely. Young brown dwarfs are similar to very low massive stars. Older brown dwarfs are similar to a giant Jupiter except that their surface temperature is much higher (900 K1 ). Their atmosphere contains large amounts of water vapor and methane. The size of brown dwarfs is similar to that of Jupiter. From the dynamical point of view it has to be stressed that brown dwarfs are predominantly free-floating objects like stars. They can be found by IR surveys and more than 100 objects are known. • Planets: objects with masses less than 13 MJ are classified as planets. The lower mass limit for planets is such that he has “cleaned” his orbit – there are no small particles left – and that he has spheric
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